The oven won't tell me when it has preheated. It doesn't beep and the light doesn't come on. The oven itself works although it seems to take longer for it to cook anything although this may be because I am guessing when the oven is ready when I put the food in. Then last night I had the oven set at 400 and an hour and a half after turning it on, the oven beeped and the preheat light came on.

SOURCE: Preheat taking long time

If your oven has the hidden bake element then it is probably working properly. A standard oven with an exposed bake element will usually take about 12-15 minutes to preheat. The hidden element models take a little longer but they usually heat a little more evenly.
Post back if you have any more questions.

SOURCE: Hotpoint RGB745 Gas Kitchen Range

I eventually replaced the oven ignitor. That has solved the problem.

SOURCE: Takes to long to Preheat

the burner element iside the oven(the rod that glow,s orange) is burned out,even though it glows and the oven is warm,the element is worn out and has to be replaced,unplug the oven,shutt off gas supply to oven,pull off back cover youll see the rod coming thru each end of the rod will have 1 wire unplug the wire off the clip,go back to the inside the oven follow the rod to the back youll see 2 screws holding the rod unscrew and pull rod thru,easy job and elements arent exspensive,but make sure to unplug oven,and shut off gas

SOURCE: hotpoint oven will sometimes preheat to temp,

if it's glowing but not igniting the flame in less than 40 seconds you have a lazy ignitor,change it out.open the draw unser the oven door and on the left or right side of frame not the door you'll see a tag with the model and serial number,give your parts store the model number and they'll give you the correct ignitor